A vehicle, in addition to the inherent crush characteristics of its structure, may have dedicated crash energy management structures. Their function is exclusively to dissipate energy in the event of a crash. Such dedicated structures have predetermined crush characteristics which contribute to the resulting deceleration pulse to which the occupants are subjected.
In the vehicular arts there are two known types of such dedicated crash energy management structures: those which are passive, and those which are active.
An example of a passive dedicated crash energy management structure is an expanded honeycomb celled material, which has been used to a limited degree in certain vehicles. FIG. 1 exemplifies the process of fabrication of a honeycomb-celled material. A roll 10 of sheet material having a preselected width W is cut to provide a number of substrate sheets 12, each sheet having a number of closely spaced adhesive strips 14. The sheets 12 are stacked and the adhesive cured to thereby form a block, referred to as a HOBE® (registered trademark of Hexcel Corporation) block 16 having a thickness T. The HOBE block is then cut into appropriate lengths L to thereby provide HOBE bricks 18. The HOBE brick is then expanded by the upper and lower faces 20, 22 thereof being separated away from each other, where during the adhesive strips serve as nodes whereat touching sheets are attached to each other. A fully expanded HOBE brick is composed of a honeycomb celled material 24 having clearly apparent hexagonal cells 26. The ratio of the original thickness T to the expanded thickness T′ is between 1 to 20 to 1 to 50. An expanded honeycomb celled material provides crash energy management parallel to the cellular axis at the expense of vehicular space that is permanently occupied by this dedicated energy management structure.
Typically, crash energy management structures have a static configuration in which their starting volume is their fixed, operative volume, i.e. they dissipate energy and modify the timing characteristics of the deceleration pulse by being compressed (i.e., crushing or stroking of a piston in a cylinder) from a larger to a smaller volume. Since these passive crash energy management structures occupy a maximum volume in the uncrushed/unstroked, initial state, they inherently occupy vehicular space that must be dedicated for crash energy management—the contraction space being otherwise unstable. Expressed another way, passive crash energy management structures use valuable vehicular space equal to their initial volume which is dedicated exclusively to crash energy management throughout the life of the vehicle even though a crash may never occur, or may occur but once during that time span. This occupied contraction space is not available for other uses, including functions such as vehicle component inspection, servicing and repair. Spaces left open for servicing, repair and operational clearances are thus locations in which passive dedicated crash energy management devices have typically not been used.
Active crash energy management structures have a predetermined size which expands at the time of a crash so as to increase their contribution to crash energy management.
One type of dedicated active crash energy management structure is a stroking device, basically in the form of a piston and cylinder arrangement. Stroking devices have low forces in extension and significantly higher forces in compression (such as an extendable/retractable bumper system) which is, for example, installed at either the fore or aft end of the vehicle and oriented in the anticipated direction of crash induced crush. The rods of such devices would be extended to span the previously empty spaces upon the detection of an imminent crash or an occurring crash (if located ahead of the crush front). This extension could be triggered alternatively by signals from a pre-crash warning system or from crash sensors or be a mechanical response to the crash itself. An example would be a forward extension of the rod due to its inertia under a high G crash pulse. Downsides of such an approach include high mass and limited expansion ratio (1 to 2 rather than the 1 to 20 to 1 to 50 possible with a compressed honeycomb celled material).
Another type of active dedicated crash energy management structure is inflatable airbags or pyrotechnic air cans. Downsides of such systems include low force levels and low ratios of crush force to added mass due to the lack of mechanical rigidity of these systems.
Accordingly, what remains needed in the vehicular arts is a dedicated vehicular crash energy management structure which provides at times other than a crash event open spaces for other uses than crash pulse management, a high level of compression ratio, high crush force, and a low crush force to mass ratio.